1. Field of the Invention
The present invention relates to a wire electric discharge machine, and in particular, relates to a wire electric discharge machine capable of starting discharge machining even if a machining gap between a wire electrode and workpiece is in a short-circuit state when machining is started.
2. Description of the Related Art
A wire electric discharge machine is devised such that if machining is started in a state in which a wire electrode and workpiece are not short-circuited, that is, in an open state when machining is started by passing a wire electrode through a minor-diameter machining start hole of the workpiece, machining can be performed in a stable manner without a high-energy discharge current flowing at once due to the presence of a resistant working fluid, for example, resistivity-controlled water or oil in a machining gap between the wire electrode and workpiece and also without fusing (breaking of wire electrode) of the wire electrode with the long-lasting discharge current limited by the resistance.
On the other hand, as shown in FIG. 16A, when machining is started, an outer circumferential surface of a wire electrode 1 may be in contact with, that is, short-circuited with an inner surface of a minor-diameter machining start hole 3 due to a positional displacement of the wire electrode 1 with respect to the center of the small-diameter machining start hole 3. It is well known that when machining is started in a short-circuit state and a power supply voltage is applied and a discharge current is supplied to a machining gap in a wire electric discharge machine, a current flows at once from workpiece 2 to the wire electrode 1 (or from the wire electrode 1 to the workpiece 2) and the wire electrode 1 is fused due to Joule heat generated by the resistance of the wire electrode 1. Therefore, the application of a power supply voltage and the supply of a discharge current are normally avoided in a short-circuit state.
To cope with such a problem, for example, JP 03-287315 A discloses a technology that, if a short-circuit state is detected when machining is started, eliminates the short-circuit state when machining is started by a wire electric discharge machine by repeating relative micro-distance movement to search for a position where the short-circuit state is eliminated.
However, the above technology has the problem described below.
Unfortunately, if the small-diameter machining start hole 3 of the workpiece 2 is minute and close to the wire diameter of the wire electrode 1, the small-diameter machining start hole 3 is formed so as to extend obliquely as shown in FIG. 16B, instead of being perpendicular to the workpiece 2, and thus, the wire electrode 1 and the small-diameter machining start hole 3 are not parallel and the wire electrode 1 and the upper edge or the lower edge of the workpiece 2 are in contact, there is a burr 4 on the inner surface of the small-diameter machining start hole 3 as shown in FIG. 16C, or the wire electrode 1 is erected not vertically, but obliquely as shown in FIG. 16D, the possibility of being able to eliminating a short-circuit state is small even if the technology described in JP 03-287315 A is used.